To remove dust stuck to a dustproof cover and prevent cooling efficiency of an radiator or an engine from deteriorating by providing the dustproof cover preventing invasion of dust in front of the radiator, and providing a changeover means leading exhaust gas in front of the dustproof cover.

A radiator 1 is fixed on an under carriage 5 outside a radiator fan 18, and further a dustproof cover 2 preventing invasion of dust is provided outside the radiator 1. A muffler 19 is provided on the upper part of an engine 3, and it is connected to an exhaust pipe 20. A changeover valve is provided on the way of the exhaust pipe 20. The lower stream side end part in the exhaust gas sending direction of a guide pipe 22 is arranged in front of the dustproof cover 2, and it is constituted so that exhaust gas is diffusively discharged in front of the outside face of the dust protective cover 2. Hereby, dust stuck to the dustproof cover 2 can be removed, and hence cooling efficiency of the radiator 1 or the engine 3 can be prevented from deteriorating.